1. Field
The present disclosure generally relates to systems and methods of actuation, and, in particular, relates to the actuators utilizing memory alloy wire.
2. Description of the Related Art
Providing secure storage frequently requires a container with a lid that is released only after certain requirements are met, such as verification that the individual accessing the container is authorized to do so. Systems of this type use an actuator of some type to release a latch that otherwise retains the lid in the closed position. Common actuators include solenoids and motors, both of which may be relatively large compared to the usable volume of the container, which decreases the volumetric efficiency of the container.
Actuators that utilize memory alloy wire can provide sufficient power and stroke to release the latch of a secure container while occupying less volume than a solenoid or motor. Memory alloy wire, also known as “muscle wire,” is made from one of a number of alloys that contract in length when heated as the crystalline structure of the metal changes from its martensite form to its austenite form. Memory alloy wire can be stretched back to its original length as it cools to its original temperature and returns to its martensite form. Example alloys include nickel-titanium alloys that were first developed by the US Naval Ordnance Laboratory and commercialized under the trade name Nitinol (taken from the words Nickel Titanium Naval Ordnance Laboratories). The memory alloy wire is commonly heated by passing an electric current through the wire, creating heat within the wire due to the internal resistance of the wire.
When used as an actuator, a bias spring is often employed to preload the memory alloy wire and stretch the wire in the cold condition. The force of the preload reduces the amount of force that can be applied by the actuator. Loss of the preload force during the operational cycle may decrease the lifetime of the actuator.
U.S. Pat. No. 6,116,461, Method and Apparatus for the Dispensing of Drugs, Broadfield et al., discloses an Automated Dispensing Machine (ADM) that utilizes a memory alloy wire actuator. While this system was a significant advance in the dispensing of medications, the memory wire is preloaded by a linear spring that applies an increasing amount of force to the memory alloy wire as the actuator moves, reducing the amount of force that can be applied by the actuator.